The present invention relates generally to valves and more specifically to an improved actuator.
Actuators generally include a force producing section which is transmitted through a coupler to the valve shaft to open and close the valve. The valve actuator is usually mounted on top of the valve housing to which it is to operate upon. The force producing section is usually connected to the coupling shaft by a link arm. In one type of device, the link arm is unitary to the coupling shaft. A typical example are U.S Pat. Nos. 2,954,754, 3,650,506 and 3,929,024. In each of these patents, the valve shaft or an adapter is received in a keyed passage in the coupling shaft.
A second type of coupling shaft and link arm wherein the link arm is mounted to the coupling shaft using a key and a key way are illustrated in U.S. Pat. Nos. 3,160,077; 3,737,142 and 3,985,151. The interior of the link arm and the exterior of the coupling shaft must be precisely machined for their interconnection to prevent any rotational displacement causing hysteresis between the motion of the link arm and the coupling shaft. The importance of the accuracy of the machining increases the cost of the actuator and does not allow for subsequent compensation due to wear.
Where the link arm is unitary to the coupling shaft, a two piece housing must be used. This is substantially more expensive since separate machining steps must be used for each half of the housing. Also more care must be taken to assure alignment of the aperatures in the housing which receive the coupling shaft.
When the link arm is separate from the coupling shaft, the housing can be unitary. This allows a reduction in machining since a single boring will form two aligned aperatures in opposed walls. Some method must be provided to mount the link arm to the coupling shaft after it has been inserted into the housing. The prior art patents use key--key ways or polygram cross-section to interconnect the link arm and coupling shaft. These types of connections produce lost motion with continued uses resulting in undesirable position errors of the valve. A press fit is one solution, but this requires expensive machining and does not allow disassembly for maintenance.